The charger has a great influence on the service life of Ni-Mh rechargeable batteries, many finished chargers, whether it is timed constant current. Detect the voltage difference. In the way of battery temperature rise, it is often found that the battery is charged with severe heat and overcharge phenomenon, which affects the service life of the battery. The timing constant current type determines the timing time according to the battery capacity and the charging current. However, the capacity of the battery after the user uses it cannot be determined, and therefore, it is easy to cause overcharging. The general automatic detection fast charge charger, because of the large charging current, has a large voltage drop and heat on the internal resistance of the battery, thus causing damage to the battery.
In summary, a charger for the Ni-Mh battery is designed to use a constant voltage control method to charge the charging current in four stages with constant currents of 410 mA, 200 mA, 85 mA, and 28 mA. As the charging current decreases, the ground will be charged to the maximum capacity. Moreover, after completion of the above four stages, the battery is continuously trickle charged with a small current of 5 mA to ensure that its capacity does not decrease over a long period of time. The rechargeable battery can be two rechargeable batteries or four batteries f connected in series, and then the two groups of batteries in parallel, the charging current of each group will be halved). The key to the circuit is how to determine the end of battery charging and its maximum charging voltage setting.
For example, the electromotive force of each Ni-Mh battery is 1.315V. Considering the internal resistance of the battery itself, the highest value of the charging voltage should be l.43~1.5V. When the battery is used for a long time, the voltage (electromotive force) of the battery does not become low, but the electrolyte is exhausted, the internal resistance is increased, and the ability to output electric power is lowered. So whether the battery is charged or not, the final judgment is still voltage. The two batteries are connected in series, with the highest charging voltage controlled at 3V.
See the attached figure for setting the charging operation method. At the beginning of the constant current fast charge phase (charge current 410mA), the voltage of the two battery strings is less than the maximum charge voltage (3V) set by the start of charging, and the (3) pin of IC555 outputs a low level. V1~V4 are turned on. The charging current is constant at around 420mA, when the voltage across the battery gradually rises to the highest voltage set by charging (about 3V). The collectors of v5 and V6 become low. The 555 chip flips for the first time (3) the pin outputs a high level), triggers the unidirectional thyristor SCR4 to turn on, and V4 turns off. In the second stage of charging, the charging current is constant at about 200 mA. Due to the sudden decrease of the charging current, the voltage across the rechargeable battery will fall, causing the 555 chip to reset again (3) the foot output low level), and the remaining three The charging circuit is still on and the charger will charge the battery at 200mA. The battery voltage will gradually rise to the highest voltage set by the charge. Then the circuit flips again, triggering SCR3. Turn V3 off. Repeat the above process, V2, V4 will trigger the shutdown. At the end of the four phases of charging, the charging circuit continues to vortex charge the battery with a small current. This small current does not cause damage to the battery and maintains the battery's charge.
Dl, ZI, R22 can set the maximum charging voltage of the battery. When the resistance of R22 increases, the maximum voltage value will decrease, and vice versa. R16, R11, R7, and R24 are the values ​​of the charging current that limit each stage. R18 ~ R20, C2 ~ C4 play a delay role, to ensure that V1 - V4 trigger flip one by one.
L1—L4 is the charging indication, and when the corresponding LED is on, it indicates that the corresponding segmentation charging is in progress. When L1~L4 is extinguished. When L5 is lit, it indicates that the charging is completed and the charger enters the turbulent hold state.
The charger's fast charge current control in the first stage is about 410mA, and it can be used for batteries with a capacity of 1000O~2500mA. However, due to the four-stage approaching charging method, the charging process of the four segments is completed and it takes a long time. In actual use, after charging to the second stage, the charging capacity has reached 95% or more, and the battery can be used normally.
Under normal circumstances. Section 2-3 stage charging is sufficient, and the corresponding line can be removed for simplification. 
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